Though wind-scoured and virtually barren, the southern Sahara Desert turns out to be a surprising sustainer of life an ocean away—in South America’s Amazon Rainforest. By studying NASA satellite data of the spread of dust across the globe, scientists discovered that more than half of the mineral dust that fertilizes the Amazon soil comes from a single spot in the southern Sahara, a large mountain-rimmed valley called the Bodele Depression.
This photo-like image from NASA’s Aqua satellite on January 2, 2007, shows a dust storm brewing in the valley. A bright streak of dust arcs southwest across the Bodele Depression toward Lake Chad. Dust veils the lower elevations, with the higher elevations of the Jos Plateau and the Adamaoua Mountains peaking out as if through fog. During the Northern Hemisphere winter months, northeasterly winds (Harmattan winds) routinely blow across this part of northern Africa. A gap between the Tibesti and Ennedi Mountains creates a natural wind tunnel that focuses and intensifies the winds across the Bodele Depression. The dust spreads westward across the Atlantic on the easterly trade winds to the Amazon, where it replenishes mineral nutrients that are continually depleted from the soil by the heavy, tropical rains.
Based on satellite data and models, scientists estimate that dust storms such as the one pictured here generate about 0.7 million tons of dust on average during winter days. About half of the 40 million tons of dust that are swept across the Atlantic from the Sahara to the Amazon each year come from the Bodele Depression, a small valley that accounts for only 0.2 percent of the entire Sahara and is only 0.5 percent the size of the Amazon itself. The discovery of this surprisingly large single source of mineral dust raises many fascinating questions about how far-flung parts of the Earth system are connected, including how large the dust reservoir in the Bodele depression is, how long it has been emitting such a huge amount of dust, and how long will it continue to fertilize the Amazon.
The large image provided above has a spatial resolution (level of detail) of 500 meters per pixel. The MODIS Rapid Response Team provides twice-daily subsets of northern Africa in additional resolutions via a clickable map.
- Koren, I., Kaufman,Y., Washington, R., Todd, M., Rudich, Y., J Vanderlei, M., and Rosenfeld, D. (2006). The Bodele depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest, Environmental Research Letters, 1 (014005), p. 1-5. doi:10.1088/1748-9326/1/1/014005. Available online from Institute of Physics Electronic Journals.
NASA image courtesy the MODIS Rapid Response Team, Goddard Space Flight Center